The present invention relates to an infrared lamp for use in heating apparatuses and the like, and more particularly to an infrared lamp using a long-size heating element formed of a sintered body including a carbon-based substance, a method of producing the infrared lamp, and a heating apparatus using the infrared lamp.
Among heating apparatuses using the infrared lamp of the present invention, there are apparatuses for heating objects by using a heat source, that is, heating apparatuses (for example, an electric stove, a kotatsu (Japanese traditional leg and feet warming apparatus), an air conditioner, an infrared medical apparatus, etc.), drying apparatuses (for example, a clothing drier, a bedding drier, a food drier, a garbage treatment apparatus, a heating-type deodorizing apparatus, etc.). The heating apparatuses further include cooking apparatuses (for example, an oven, an oven range, an oven toaster, a toaster, a roaster, a heat retaining apparatus, a yakitori cooker (skewered chicken cooker), a cooking stove, a defroster, etc.,) hairdressing apparatuses (for example, a drier, a permanent wave heater, etc.). The heating apparatuses still further include apparatuses for fixing letters, images, etc. on sheets (apparatuses for carrying out display by using toner, for example, LBP, PPC and facsimile, and apparatuses for thermal transfer of a printed film onto an object by heating).
A tungsten wire or a nichrome wire has been principally used as the heating element of a conventional infrared lamp. Since the tungsten wire is oxidized in the air, the tungsten wire is enclosed in a quartz glass tube or the like, and the quartz glass tube is filled with an inert gas. A lamp-type heating element is produced in the above-mentioned way.
As a heating element formed of the nichrome wire, a coil-shaped nichrome wire inserted into an opaque quartz glass tube or the like for protection is produced so as to be used in the air. The electric resistance of the tungsten wire is lower in unlit state of the lamp than that in lit state, and therefore a large rush current flows at the time of turning on of the lamp. Such a rush current may adversely affects peripheral apparatuses. Furthermore, the nichrome wire has a problem of slow temperature rising speed. To solve these problems, heating elements made of carbon-based substances have been developed.
For example, Japanese Laid-open Patent Application No. Hei 10-859526 discloses a heating element formed of a sintered body made of a carbon-based substance including carbon and a metallic or semi-metallic compound (metallic carbide, metallic nitride, metallic boride, metallic silicide, metallic oxide, semi-metallic nitride or semi-metallic carbide). Accordance to an embodiment of the above-mentioned Laid-open patent application, natural graphite powder, boron nitride and a plasticizer are added to the mixture resin of a chlorinated vinyl chloride resin and a furan resin, and these ingredients are dispersed by a Henschel mixer. The ingredients are then kneaded by two rollers and pelletized by a pelletizer. Pellets obtained in this way are extruded by a screw-type extruder in the shape of a rod. The rod is dried and then fired in a nitrogen gas. Since the emissivity of carbon is close to that of a black body, it is assumed that a heating element formed of a sintered body including a carbon-based substance is an ideal heating element for the light radiation. A pure carbon material invented by Edison is known as a conventional heating element formed of carbon. However, since the carbon has a low inherent resistance, it is difficult to obtain a heating element having a high resistance. The above-mentioned prior art uses materials obtained by mixing carbon with a metallic or semi-metallic compound and by firing the mixtures. Materials obtained by this method have inherent resistances larger than that of pure carbon by several times to several ten times. An infrared lamp using a heating element formed of a sintered body including such a carbon-based substance is disclosed in Japanese Laid-open Patent Application No. Hei 11-54092. The structure of the infrared lamp is described below referring to FIG. 13, a fragmentary sectional view.
Referring to FIG. 13, a coil-shaped section 32 formed at one end of an internal lead wire 31 made of tungsten is tightly wound around one end of a resistance heating element 1 formed of a carbon-based substance. Another coil-shaped section 33 is formed in the middle of the internal lead wire 31. The other end of the internal lead wire 31 is welded to one end of a molybdenum foil 6. An external lead wire 7 is welded to the other end of the molybdenum foil 6. A metallic sleeve 34 made of an alloy of iron and nickel is fastened and fixed around the coil-shaped section 32.
There is no description regarding the temperature rise and electric resistance of the heating element formed by sintering the mixture of a carbon-based substance and a metallic or semi-metallic compound, in the Japanese Laid-open Patent Application No. Hei 10-859526. That is, a resistance-temperature characteristic thereof is not disclosed. The heating element used: for the infrared lamp disclosed in the afore-mentioned Japanese Laid-open Patent Application No. Hei 11-54092 has a negative resistance-temperature characteristic wherein its electric resistance lowers as the temperature rises. Therefore, no rush current flows at the time of turning on.
However, the afore-mentioned Japanese Laid-open Patent Application No. Hei 11-54092 does not disclose any examples of the resistance-temperature characteristic value. The resistance-temperature characteristic of a heating element is a very important factor when producing a heater. In other words, when the resistance-temperature characteristic value is unstable, it is necessary to check the characteristic value in each production lot and to change the cross-sectional area or the heating length of the heating element according to the characteristic value. The necessity of these kinds of works make impossible the mass production of infrared lamps. When heaters having a stable resistance-temperature characteristic value are produced, its absolute value is also important. In other words, no rush current flows when the electric resistance in lit state is smaller than the electric resistance in unlit state. However, since the resistance decreases as the temperature of the heating element rises, a dangerous state in which the current increases and temperature rise further is liable to occur. In other words, when the heating element deteriorates during use, this may bring a danger of decreasing the resistance further. On the other hand, when the electric resistance in lit state is high, there is no problem when the electric resistance is relatively low. However, when the electric resistance increases, the rush current flows, and there is the same problem as that in the case of the conventional lamp using a tungsten wire. FIG. 14 is a sectional view showing an infrared lamp in accordance with another prior art.
Referring to FIG. 14, internal leads 104 extended from both ends of a heating element 120 formed of a coiled tungsten wire are welded to metallic foils 105 serving as intermediate terminal plates, thereby producing a heating element assembly 120a. This heating element assembly 120a is inserted into a quartz glass tube 101. Both ends of the quartz glass tube 101 are melted and the quartz glass tube 101 is filled with an inert gas and sealed at the metallic foils 105, thereby producing an infrared lamp.
The coil-shaped heating element 120 has a uniform radiation intensity distribution in a direction perpendicular to the axis of the coil. Therefore, it is necessary to install a reflector or the like when the heating element 120 is used for a heating apparatus for generating radiant heat in one direction. The coil-shaped heating element 120 has a hollow portion inside the coil, and clearances are present between the wires of the coil. Hence, surplus energy is consumed to radiate heat to the space.
To solve these problems, the above-mentioned Japanese Laid-open Patent Application No. Hei 11-54092 discloses another conventional infrared lamp. This infrared lamp uses a wire-shaped heating element formed of a sintered body including a carbon-based substance instead of the conventional coil-shaped heating element 120.
In the infrared lamp disclosed in the above-mentioned. Japanese Laid-open Patent Application No. Hei 11-54092, since the heating element including the carbon-based substance is used, the infrared ray emissivity of the heating element has a high value ranging from 78 to 84%. In other words, the infrared emissivity is increased by using the sintered body including the carbon-based substance as a heating element. In addition, since the heating element is wire-shaped, surplus energy released to an internal space in the case of the conventional coil-shaped heating element is not consumed. Furthermore, when the heating element is made plate-shaped, directivity can be offered to the thermal radiation intensity distribution thereof.
The infrared lamp disclosed in the above-mentioned Japanese Laid-open Patent Application No. Hei 11-54092 has the following problems.
When a heating element is made long, the long heating element is liable to hang down due to its own weight during heating. Furthermore, when the length of the heating element exceeds a certain value, pressure application during forming process may become nonuniform or may bend during sintering. Hence, the production yield of the heating element becomes low and the production cost thereof rises. It is thus difficult to form a long heating element.
Furthermore, it is also difficult to change the thermal distribution of the heating element.